The Parthians emerged around 238 b.c. as a coalition of tribes in eastern Iran led by a king named Arsaces (hence the dynasty is also referred to as the Arsacids). They moved into western Iran during the reign of their king, Mithridates I (171–138 b.c. ). Their empire lasted longer than any other Iranian empire, but it is the least well understood because of a lack of historical sources from that period. Although the Parthians were quite partial to Greek culture (calling themselves Hellenophiles on their coinage), they were relentlessly anti-Roman in their foreign policy; it is the history of their wars, as recorded by Roman sources, that makes up most of what is known about them. Their most famous battle was with the Roman general Crassus, whose forces were annihilated at Carrhae in 53 b.c.
The Sasanid Dynasty was established as the result of an epic and somewhat mysterious adventure that recalls in some ways the rise of Cyrus. Around a.d. 224, Ardashir, the leader of a priestly family in the province of Persia (Persis; Fârs), overthrew and killed the Parthian governor. He then exploited a dispute over the accession to the Parthian throne and, by a.d. 228, defeated the Parthian king Artabanus V. Through a tireless series of military campaigns, Ardashir and his successors effectively recreated the borders of the Persian Empire as it had existed under the Achaemenids; the early Sasanid rulers were consequently preoccupied with the task of defending those borders against the Romans and Byzantines in the West and the Hephthalites and other nomads in the East, as well as beating back occasional challenges to their rule by Arsacid pretenders. The dynasty reached its peak under Khosrow I Anushirvan, known as the Just ( a.d. 531–79), and Khosrow II Parviz, the Victorious ( a.d. 591–628). Khosrow I suppressed a major socio-religious uprising (the Mazdakites), reorganized the fiscal and administrative structure of the empire, promoted a cultural revival, and gave refuge to pagan Greek philosophers fleeing oppression by the Christian Byzantine rulers. Khosrow II, who also had to put down a major rebellion by a general from a Parthian family, supposedly dreamed of becoming ruler of the world, an ambition symbolized by keeping two empty thrones below his own for the kings of Byzantium and China whom he hoped to make his vassals. He hardly posed much of a threat to China, but he did engage in a major war with the Byzantines, capturing Jerusalem in a.d. 614, occupying Egypt in a.d. 616, and reaching the Bosporus in a.d. 617. Nonetheless, this adventure ended in disaster as the empire’s finances were exhausted, the Arab subjects in Mesopotamia rebelled, and the Byzantines mounted a counteroffensive which threatened the Sasanid capital in a.d. 627. The next year, Khosrow II was deposed and executed by members of his own family, plunging Iran into chaos and setting the stage for the invasion and destruction of the empire by the armies of Islam.